Update some Phabricator behaviors for changes to Futures

Summary:
Depends on D21053. Ref T11968. Three things have changed:

• Overseers can no longer use FutureIterator to continue execution of an arbitrary list of futures from any state. Use FuturePool instead.
• Same with repository daemons.
• Probably (?) fix an API change in the Harbormaster exec future.

Test Plan:

• Ran "bin/phd debug task" and "bin/phd debug pull", no longer saw Future-management related errors.
• The Harbormaster future is easiest to test by just seeing if production works once this change is deployed there.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21054